In modern motor vehicles, automated clutches, i.e., clutches operated by an actuator, are becoming increasingly common. In most cases, the position of an actuating element of the clutch and/or of the actuator, is determined using an incremental counter that immediately provides information only on the actuating paths that have just been traveled through, but not on the absolute position of the actuating element. To detect the absolute position, a referencing process is required wherein the actuator moves the clutch into a predetermined actuating position and the respective counter reading of the incremental counter is read out.
A fundamental problem of such incremental path measurements is their dependence on the exact determination of the reference position. This problem is intensified by the fact that during the operation of a vehicle, malfunctions are to be expected that may cause the absolute position of the clutch actuator and/or of the actuating element of the clutch and thus the clutch torque set by the actuator to become unknown to the electronic control device. As a result, safety-critical events may occur—for instance, the vehicle may start to move without the driver's intention. An exact knowledge of the respective operating position of the clutch is particularly safety-relevant in a twin clutch transmission in which two clutches need to be operated in a precisely coordinated manner relative to each other.
Such twin clutch transmissions are used in motor vehicles as power shift transmissions. Power shift operation is possible since every partial drive train can be operated separately and independently of the second partial drive train. Thus in one partial drive train the clutch may be closed and a gear may be engaged to operate the vehicle. During this operation, a gear having a different transmission ratio may be engaged in the second partial drive train. To shift gears, the clutch of the partial drive train that is currently in operation is opened while the clutch of the partial drive train that is to be activated is being closed. During such a process, which may be referred to as a period of overlap or an overlapping shift, both clutches transmit a torque to their partial drive trains. To prevent the power take-off from locking as a result of the different transmission ratios of the two gears engaged in the two partial drive trains, the sum of the transmittable torques on both clutches must not exceed the torque to be transmitted (e.g., the engine torque) by any large amount.
The contact points of the clutch are determined at the initial start of operation and are then adapted as required during the operation of the vehicle.
The adaption of the clutch parameters, in particular of the contact points, by evaluating the torques of a drive train monitoring element during operation is described in German Patent Document No. 10213946 A1.
German Patent Document No. 10 2008 023 360 A1 discloses to determine the contact point by bringing the input shaft of the clutch in question to a predetermined rotational speed by closing the clutch. When the clutch is opened, the rotational speed will decrease. Upon re-closing the clutch, the speed gradient can be evaluated. At the instant at which the gradient changes significantly, the clutch starts to transmit a torque, and the current clutch position is interpreted as the contact point.
European Patent Document No. 1 067 008 B1 discloses to directly draw conclusions on the torque that acts on the rotational body “input shaft” by disengaging the gear and evaluating the speed gradient of the input shaft. At the instant of disengagement of the gear, the clutch has already been closed to a certain torque value. The torque resulting from the speed gradient and the known mass inertia of the input shaft will always be falsified by a drag torque (for instance caused by bearing friction). To determine the drag torque, the process is repeated and the gear is disengaged with the clutch open. Compared to the method disclosed in DE 10 2008 023 360 A1, the process disclosed herein always starts at the synchronization speed of the input shafts. This is necessary because the method is tailored to wet clutches in which the drag torques are highly dependent on the slip speed.
A disadvantage of detecting the clutch contact point in accordance with the method disclosed in EP 1 067 008 B1 is that the gears are frequently engaged and disengaged—in some cases even against the action of a clutch torque. The synchronization devices involved in the process are thus of a more robust design to be able to withstand the increased wear. Another disadvantage is that the shift program cannot change to the shaft that has so far been inactive when the gear is disengaged to determine the contact point. Thus the method cannot be repeated ad libitum without affecting the shifting process.
German Patent Document No. DE 10 2007 025 501 A1 discloses to close the clutch when the gear is disengaged. At the instant at which the transmission input shaft speed changes very quickly towards the engine speed, the current clutch position is interpreted as the contact point. In contrast to the method disclosed in DE 10 2008 023 360 A1, this document does not make any suggestion as to how to deal with an input shaft that co-rotates due to drag torques or the overcoming of static friction.
Experiments have shown that the speed of the input shaft will not always decrease on the shaft in question when the clutch is open and the neutral gear engaged. In many cases, the input shaft will continue to co-rotate with the engine due to drag torques of the clutch bearing. Friction occurs in the bearings between the two input shafts of the two partial transmissions, which are embodied as hollow shaft and massive shaft.